Magnetic disk device capable of correcting servo demodulation position

ABSTRACT

According to one embodiment, a magnetic disk device includes a disk including two first servo sectors and at least a second servo sector, a head, and a controller, wherein the first servo sector includes burst data and a first data pattern written before the circumferential direction of the burst data, the second servo sector includes the burst data, the first data pattern, and a second data pattern written after the circumferential direction of the burst data, a first frequency of the first data pattern is different from a second frequency of the second data pattern, and a first length of the first data pattern is different from a second length of the second data pattern.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefit of priority fromJapanese Patent Application No. 2019-042706, filed Mar. 8, 2019, theentire contents of which are incorporated herein by reference.

FIELD

Embodiments described herein relate generally to a magnetic disk device,a writing method of a servo sector, and a method of correcting a servodemodulation position.

BACKGROUND

A magnetic disk device having a short servo sector whose circumferentiallength is shorter than a circumferential length of a normal servo sectorhas been considered. The magnetic disk device sequentially demodulates apreamble, a servo mark, a gray code, burst data, and a post code in anormal servo sector. The magnetic disk device demodulates only burstdata in a short servo sector. Servo data read in the short servo sectoris less than servo data read in the normal servo sector. Therefore, inthe magnetic disk device, for example, the read timing cannot besynchronized by servo mark, the read timing changes, and the quality ofthe demodulation process of the short servo sector may be degraded. Inaddition, burst data is written in a data pattern whose phase isinverted by 180° in one servo track cycle in the radial direction of thedisk. Therefore, when burst data is read in the short servo sector, itmay be difficult to determine whether the read timing deviates ordeviates in the radial direction.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram illustrating a configuration of a magneticdisk device according to an embodiment.

FIG. 2 is a schematic diagram illustrating an example of the arrangementof a normal servo and a short servo according to an embodiment.

FIG. 3A is a schematic diagram illustrating an example of aconfiguration of a normal servo according to an embodiment.

FIG. 3B is a schematic diagram illustrating an example of aconfiguration of a short servo according to an embodiment.

FIG. 3C is a schematic diagram illustrating an example of aconfiguration of a normal servo according to an embodiment.

FIG. 4A is a diagram illustrating an example of a process ofdemodulating a particular normal servo of a particular track.

FIG. 4B is a diagram illustrating an example of a process ofdemodulating a particular short servo of a particular track.

FIG. 5 is a diagram illustrating an example of a process of demodulatingN burst, Q burst, and additional pattern of a short servo in a shortservo mode.

FIG. 6 is a diagram illustrating an example of a phase of eachadditional pattern corresponding to each circumferential position of aparticular track demodulated in a normal servo mode and a phase of eachadditional pattern corresponding to each circumferential position of aparticular track demodulated in a short servo mode.

FIG. 7 is a diagram illustrating an example of a path of a servodemodulation position in which the servo demodulation position iscorrected in a short servo mode when a head deviates in a radialdirection at a particular circumferential position.

FIG. 8 is a flowchart illustrating an example of a method of correctinga servo demodulation position according to an embodiment.

FIG. 9 is a flowchart illustrating an example of a process of writing anadditional pattern according to an embodiment.

FIG. 10 is a flowchart illustrating an example of a process of writingan additional pattern according to an embodiment.

FIG. 11 is a flowchart illustrating an example of a process of writingan additional pattern according to an embodiment.

FIG. 12 is a schematic diagram illustrating an example of aconfiguration of a short servo according to Modification 1.

DETAILED DESCRIPTION

In general, according to one embodiment, a magnetic disk devicecomprising: a disk comprising two first servo sectors arranged side byside in a circumferential direction and at least a second servo sectorlocated between the two first servo sectors; a head that writes data tothe disk and reads data from the disk; and a controller that demodulatesall data of the first servo sector and demodulates data of part of thesecond servo sector, wherein the first servo sector comprises burst dataand a first data pattern written before the circumferential direction ofthe burst data, the second servo sector comprises the burst data, thefirst data pattern written before the circumferential direction of theburst data, and a second data pattern written after the circumferentialdirection of the burst data, a first frequency of the first data patternis different from a second frequency of the second data pattern, and afirst length in the circumferential direction of the first data patternis different from a second length in the circumferential direction ofthe second data pattern.

Hereinafter, embodiments will be described with reference to thedrawings. Note that the drawings are merely examples and do not limitthe scope of the invention.

EMBODIMENT

FIG. 1 is a block diagram illustrating a configuration of a magneticdisk device 1 according to an embodiment.

The magnetic disk device 1 includes a head disk assembly (HDA) to bedescribed later, a driver IC 20, a head amplifier integrated circuit(hereinafter referred to as a head amplifier IC or a preamplifier) 30, avolatile memory 70, a nonvolatile memory 80, a buffer memory (buffer)90, and a system controller 130 that is a 1-channel integrated circuit.In addition, the magnetic disk device 1 is connected to a host system(hereinafter simply referred to as a host) 100.

The HDA includes a magnetic disk (hereinafter referred to as a disk) 10,a spindle motor (hereinafter referred to as an SPM) 12, an arm 13 onwhich a head 15 is mounted, and a voice coil motor (hereinafter referredto as a VCM) 14. The disk 10 is attached to the SPM 12 and is rotated bythe driving of the SPM 12. The arm 13 and the VCM 14 constitute anactuator. The actuator performs movement control such that the head 15mounted on the arm 13 is moved to a particular position of the disk 10by driving the VCM 14. The disk 10 and the head 15 may be provided intwo or more numbers.

In the disk 10, a user data area 10 a available from a user and a systemarea 10 b for writing information necessary for system management areallocated in a region to which data can be written. Hereinafter, thedirection orthogonal to the radial direction of the disk 10 is referredto as a circumferential direction. In addition, a particular position ofthe disk 10 in the radial direction may be referred to as a radialposition, and a particular position of the disk 10 in thecircumferential direction may be referred to as a circumferentialposition. The radial position corresponds to, for example, a track, andthe circumferential position corresponds to, for example, a sector. Theradial position and the circumferential position may be collectivelyreferred to simply as a position.

The head 15 includes a slider as a main body, and includes a write head15W and a read head 15R that are mounted on the slider. The write head15W writes data to the disk 10. The read head 15R reads data recorded inthe track on the disk 10. Note that the write head 15W may be simplyreferred to as the head 15, the read head 15R may be referred to simplyas the head 15, and the write head 15W and the read head 15R may becollectively referred to as the head 15. The central portion of the head15 may be referred to as the head 15, the central portion of the writehead 15W may be referred to as the write head 15W, and the centralportion of the read head 15R may be referred to as the read head 15R.The “track” is used as one of a plurality of radially divided regions ofthe disk 10, data extending in the circumferential direction of the disk10, data written to the track, or various other meanings. The “sector”is used as one of a plurality of circumferentially divided regions ofthe track, data written to a particular position of the disk 10, datawritten to a sector, or other various meanings. In addition, the radialwidth of the track is referred to as a track width, and the centralposition of the track width is referred to as a track center.

FIG. 2 is a schematic diagram illustrating an example of the arrangementof a normal servo and a short servo according to an embodiment. Asillustrated in FIG. 2, the direction toward the outer circumference ofthe disk 10 in the radial direction is referred to as an outwarddirection (outward), and the direction opposite to the outward directionis referred to as an inward direction (inward). In addition, FIG. 2illustrates the rotational direction of the disk 10. Note that therotational direction may be a reverse direction.

The disk 10 has a plurality of servo regions SV. Hereinafter, the servoregion SV may be referred to as a servo sector. The plurality of servoregions SV extend radially in the radial direction of the disk 10 andare discretely arranged at particular intervals in the circumferentialdirection. A recording area for writing user data and the like isarranged between two servo regions SV continuous in the circumferentialdirection. The servo region SV has, for example, a servo region NSV(hereinafter referred to as a normal servo) and a servo region(hereinafter referred to as a short servo, a short servo sector, or ashort servo area) SSV different from the servo region NSV. The length ofthe data pattern in the circumferential direction of the short servo SSV(hereinafter also simply referred to as the length) is shorter than thelength of the normal servo NSV. In the example illustrated in FIG. 2,the normal servo NSV and the short servo SSV are alternately arranged inthe circumferential direction. In other words, one short servo SSV isarranged between two continuous normal servos NSV in the circumferentialdirection. Note that two or more short servos SSV may be arrangedbetween two continuous normal servos NSV in the circumferentialdirection.

FIGS. 3A and 3C are schematic diagrams illustrating an example of theconfiguration of the normal servo NSV according to the presentembodiment. FIGS. 3A and 3C illustrate a particular normal servo NSVwritten to a particular track TRn. As illustrated in FIGS. 3A and 3C, inthe circumferential direction, a direction of reading/writing isreferred to as a read/write direction. The read/write directioncorresponds to, for example, a direction opposite to the rotationaldirection illustrated in FIG. 2. The read/write direction is a directionfrom a front side to a rear side. The front side corresponds to adirection forward in time, and the rear side corresponds to a directionbackward in time. Hereinafter, the front side may be simply referred toas the front, and the rear side may be simply referred to as the rear.

The normal servo NSV includes servo data, for example, a preamble, aservo mark, a gray code, a PAD, burst data, and a post code. Note that,as illustrated in FIG. 3C, the normal servo NSV may not include the postcode. The preamble, the servo mark, the gray code, the PAD, the burstdata, and the post code are sequentially arranged in this order from thefront to the rear of the read/write direction. The preamble includespreamble information for synchronizing with a reproduction signal of theservo pattern including the servo mark and the gray code. The servo markincludes servo mark information indicating the start of the servopattern. The gray code includes an address of a particular track(cylinder address) and an address of a servo sector of the particulartrack. The burst data is data (relative position data) used to detectradial and/or circumferential positional deviation (positional error) ofthe head 15 with respect to a track center of a particular track, andincludes a repeating pattern having a particular cycle. Hereinafter, theradial position deviation (positional error) of the head 15 with respectto the track center of the particular track detected by using the burstdata is referred to as a servo demodulation position, a servo off trackposition, or a demodulation position. The PAD includes PAD informationof a synchronization signal such as a gap and a servo AGC. The burstdata is written in a data pattern in which the phase of the burst datais inverted by 180° in one servo track cycle in the radial direction ofthe disk 10. In other words, a phase of a waveform of burst datacalculated by demodulating particular burst data by, for example,discrete Fourier transform (DFT) or the like is inverted by 180° withrespect to a phase of adjacent burst data calculated by demodulatingparticular burst data adjacent in the radial direction of the burst data(hereinafter referred to as adjacent burst data). The servo track (servocylinder) corresponds to a track to be subjected to write processing orread processing according to a command from the host 100 or the like.Hereinafter, for convenience of explanation, the “phase of the waveformof the particular data calculated by demodulating the particular databy, for example, discrete Fourier transform” is simply referred to asthe “phase of the particular data”. The burst data is used, for example,to acquire the radial and/or circumferential position (hereinafter alsoreferred to as a head position) of the head 15 on the disk 10. The burstdata includes, for example, N burst and Q burst. The N burst and the Qburst are written in data patterns in which the phases deviate by 90°with each other in the radial direction of the disk 10. In other words,the phases of the N burst and the phase of the Q burst, for example,radially deviate from each other by 90°. The post code includes data(hereinafter referred to as RRO correction data) or the like forcorrecting an error caused by track distortion with respect to the trackcenter (target path) concentric with the disk 10 caused by a shake(repeated run out (RRO)) synchronized with the rotation of the disk 10when servo data is written to the disk. Hereinafter, for convenience ofexplanation, the error caused by track distortion with respect to thetrack center caused by the RRO may be simply referred to as RRO. Inaddition, the post code may also include a post code corresponding tothe short servo SSV. The frequency of the waveform of the particularpost code calculated by demodulating the particular post code by, forexample, discrete Fourier transform is equal to the frequency of thewaveform of the particular preamble calculated by demodulating theparticular preamble by, for example, discrete Fourier transform.Hereinafter, for convenience of explanation, the “frequency of thewaveform of the particular data calculated by demodulating theparticular data by discrete Fourier transform or the like” is simplyreferred to as the “frequency of data”. The phase of the post codechanges irregularly in the circumferential direction. The length PCL ofthe post code is, for example, several tens of dibits. Here, 1 dibit isthe reciprocal of the frequency of the preamble (servo preamble). Inother words, 1 dibit corresponds to the period of the waveform of theparticular preamble calculated by demodulating the preamble by, forexample, discrete Fourier transform. Hereinafter, for convenience ofexplanation, the “period of the waveform of the particular datacalculated by demodulating the particular data by discrete Fouriertransform or the like” is simply referred to as the “period of data”.When it is assumed that the frequency of the preamble is FP, 1 dibit isrepresented by 1/FP.

FIG. 3B is a schematic diagram illustrating an example of theconfiguration of the short servo SSV according to the presentembodiment. FIG. 3B illustrates a short servo SSV written to each oftracks TRn−1, TR, and TRn+1 sequentially arranged at intervals in oneservo track cycle in the radial direction.

The short servo SSV includes servo data, for example, a preamble, aservo mark, a gray code, a PAD, burst data (N burst and Q burst), and anadditional pattern. The preamble, the servo mark, the gray code, thePAD, the burst data, and the additional pattern are sequentiallyarranged in this order from the front to the rear of the read/writedirection. The length of the preamble of the short servo SSV is equalto, for example, the length of the preamble of the normal servo NSV.Note that the length of the preamble of the short servo SSV may bedifferent from the length of the preamble of the normal servo NSV. Thelength of the servo mark of the short servo SSV is equal to, forexample, the length of the servo mark of the normal servo NSV. Note thatthe length of the servo mark of the short servo SSV may be differentfrom the length of the servo mark of the normal servo NSV. The length ofthe gray code of the short servo SSV is equal to, for example, thelength of the gray code of the normal servo NSV. Note that the length ofthe gray code of the short servo SSV may be different from the length ofthe gray code of the normal servo NSV. The length of the PAD of theshort servo SSV is equal to, for example, the length of the PAD of thenormal servo NSV. Note that the length of the PAD of the short servo SSVmay be different from the length of the PAD of the normal servo NSV. Thelength of the burst data of the short servo SSV is equal to, forexample, the length of the burst data of the normal servo NSV. Note thatthe length of the burst data of the short servo SSV may be differentfrom the length of the burst data of the normal servo NSV. The length ofthe N burst of the short servo SSV is equal to, for example, the lengthof the N burst of the normal servo NSV. Note that the length of the Nburst of the short servo SSV may be different from the length of the Nburst of the normal servo NSV. The length of the Q burst of the shortservo SSV is equal to, for example, the length of the Q burst of thenormal servo NSV. Note that the length of the Q burst of the short servoSSV may be different from the length of the Q burst of the normal servoNSV. The additional pattern is data different from the post code. Thefrequency of the additional pattern is different from the frequency FPof the preamble. In other words, the frequency of the additional patternis different from the frequency of the post code. For example, thefrequency of the additional pattern is equal to the frequency of theburst data, for example, the frequency of the N burst and the frequencyof the Q burst.

For example, the frequency of the additional pattern is FP/2. The phaseof the additional pattern (first phase) periodically changes in thecircumferential direction. The additional pattern is written in the datapattern in which the phases are equal in one servo track cycle in theradial direction of the disk 10. In other words, the phase of theparticular additional pattern (first phase) is equal to the phase of theadditional pattern (first phase) radially adjacent to the particularadditional pattern (hereinafter referred to as the adjacent additionalpattern). The length APL of the additional pattern is shorter than thelength PCL of the post code. For example, when it is assumed that thefrequency of the additional pattern is FAD, the length APL of theadditional pattern is represented by the following equation.PCL>APL≥(2/FP+1/FAD)  (1)

Here, 2/FP is, for example, 2 dibits. In other words, the length APL ofthe additional pattern is equal to or longer than the sum of twice theone cycle of the preamble and the one cycle of the additional pattern.For example, the length APL of the additional pattern is 4 dibits ormore and less than the length PCL of the post code. In addition, thelength APL of the additional pattern is less than the sum SVL of thelengths up to the preamble, the servo mark, the gray code, and the PAD.SVL≥PCL>APL≥(2/FP+1/FAD)

The driver IC 20 controls the driving of the SPM 12 and the VCM 14according to the control of the system controller 130 (specifically, theMPU 60 described later).

The head amplifier IC (preamplifier) 30 includes a read amplifier and awrite driver. The read amplifier amplifies a read signal read from thedisk 10 and outputs the amplified read signal to the system controller130 (specifically, the read/write (R/W) channel 40 described later). Thewrite driver outputs, to the head 15, a write current corresponding to asignal output from the R/W channel 40.

The volatile memory 70 is a semiconductor memory in which stored data islost when power supply is cut off. The volatile memory 70 stores dataand the like necessary for processing in each unit of the magnetic diskdevice 1. The volatile memory 70 is, for example, a dynamic randomaccess memory (DRAM) or a synchronous dynamic random access memory(SDRAM).

The nonvolatile memory 80 is a semiconductor memory that records storeddata even when power supply is cut off. The nonvolatile memory 80 is,for example, a NOR type or NAND type flash read only memory (FROM).

The buffer memory 90 is a semiconductor memory that temporarily recordsdata and the like transmitted and received between the magnetic diskdevice 1 and the host 100. Note that the buffer memory 90 may beintegrated with the volatile memory 70. The buffer memory 90 is, forexample, a DRAM, a static random access memory (SRAM), an SDRAM, aferroelectric random access memory (FeRAM), and a magnetoresistiverandom access memory (MRAM).

The system controller (controller) 130 is realized by, for example,using a large scale integrated circuit (LSI) called a system-on-a-chip(SoC) in which a plurality of elements are integrated on a single chip.The system controller 130 includes a read/write (R/W) channel 40, a harddisk controller (HDC) 50, and a microprocessor (MPU) 60. The systemcontroller 130 is electrically connected to, for example, the driver IC20, the head amplifier IC 30, the volatile memory 70, the nonvolatilememory 80, the buffer memory 90, the host 100, and the like.

The R/W channel 40 performs signal processing of read data transferredfrom the disk 10 to the host 100 and write data transferred from thehost 100 in response to an instruction from the MPU 60 described later.The R/W channel 40 has a circuit or function that measures the signalquality of the read data. The R/W channel 40 is electrically connectedto, for example, the head amplifier IC 30, the HDC 50, and the MPU 60.

The HDC 50 controls data transfer between the host 100 and the R/Wchannel 40 in response to an instruction from the MPU 60 describedlater. The HDC 50 is electrically connected to, for example, the R/Wchannel 40, the MPU 60, the volatile memory 70, the nonvolatile memory80, the buffer memory 90, and the like.

The MPU 60 is a main controller that controls each unit of the magneticdisk device 1. The MPU 60 controls the VCM 14 through the driver IC 20and performs servo control for positioning the head 15. In addition, theMPU 60 controls the SPM 12 through the driver IC 20 and rotates the disk10. The MPU 60 controls the operation of writing data to the disk 10 andselects the storage destination of the write data. In addition, the MPU60 controls the operation of reading data from the disk 10 and controlsthe processing of read data. The MPU 60 is connected to each unit of themagnetic disk device 1. The MPU 60 is electrically connected to, forexample, the driver IC 20, the R/W channel 40, and the HDC 50.

The MPU 60 includes a read/write controller 610, a demodulation unit620, a correction unit 630, and a pattern writing unit 640. The MPU 60performs the processing of these units, for example, the read/writecontroller 610, the demodulation unit 620, the correction unit 630, andthe pattern writing unit 640 on firmware. Note that the MPU 60 mayinclude these units, for example, the read/write controller 610, thedemodulation unit 620, the correction unit 630, and the pattern writingunit 640 as circuits.

The read/write controller 610 controls data read processing and writeprocessing according to a command from the host 100. The read/writecontroller 610 controls the VCM 14 through the driver IC 20, positionsthe head 15 at a particular position of the disk 10, and reads or writesdata.

The demodulation unit 620 positions the head 15 (the read head 15R) at aparticular position (hereinafter referred to as a servo demodulationposition) of a servo region SV of a particular track through the R/Wchannel 40, and performs demodulation processing on read data read at aparticular timing (hereinafter referred to as a read timing).Hereinafter, the servo demodulation position in the radial direction maybe referred to as a servo radial position or simply as a servodemodulation position, the servo demodulation position in thecircumferential direction may be referred to as a servo circumferentialposition or simply a servo demodulation position, and the radial andcircumferential servo demodulation positions may be collectivelyreferred to as a servo demodulation position. Note that the demodulationunit 620 may be included in the R/W channel 40. The demodulation unit620 positions the head 15 (the read head 15R) at a target servodemodulation position (hereinafter referred to as a target servodemodulation position) calculated based on particular data of a normalservo NSV of a particular track, starts reading the normal servo NSV ata particular read timing (hereinafter referred to as a start timing),reads and demodulates from a preamble to a post code, and ends thereading of the normal servo NSV at a particular read timing (hereinafterreferred to as an end timing). Note that, when no post code is includedin the normal servo NSV, the demodulation unit 620 may read from thepreamble to the burst. The demodulation unit 620 positions the head 15(the read head 15R) at the target servo demodulation position of theshort servo SSV calculated based on the particular data of the normalservo NSV read immediately before, starts the reading of the short servoSSV at a particular start timing based on the timing when the particulardata is read by the normal servo NSV read immediately before, reads anddemodulates the N burst, the Q burst, and the additional pattern, andends the reading of the short servo SSV at a particular end timing. Theread/write controller 610, for example, performs setting such that thetime for reading the N burst, the Q burst, and the additional patternwith the servo SSV shorter than the time for reading the N burst, the Qburst, and the post code with the normal servo NSV, is shortened by thetime corresponding to a difference between the length of the post codeand the length of the additional pattern. Note that when the normalservo NSV does not include the post code, the read/write controller 610,for example, performs setting such that the time for reading the Nburst, the Q burst, and the additional pattern with the servo SSV longerthan the time for reading the N burst and the Q burst with the normalservo NSV is lengthened by the time corresponding to the length of theadditional pattern.

FIG. 4A is a diagram illustrating an example of a process ofdemodulating a particular normal servo NSV of a particular track TRn.FIG. 4A illustrates a normal servo gate (Normal SG) that demodulates allthe servo data written in the servo region SV, and Servo Mark Foundindicating the timing at which the servo mark is detected (read). TheNormal SG rises at a start timing T4A1 corresponding to the leading endof the preamble and falls at an end timing T4A2 corresponding to thetrailing end of the post code. The Servo Mark Found rises at a timingT4A3 corresponding to the rear end portion of the servo mark.

The demodulation unit 620 positions the read head 15R at the particulartrack TRn based on the preamble, the servo mark, the gray code, and thelike of the normal servo NSV of the track TRn, starts reading the normalservo NSV at the start timing T4A1, positions the read head 15R at thetarget servo demodulation position of the particular track TRn, readsand demodulates the servo mark, the gray code, the PAD, the N burst, theQ burst, and the post code in this order, and ends the reading at theend timing T4A2 at which the post code is read. The demodulation unit620 detects the timing T4A3 at which the servo mark is detected (read).For example, the demodulation unit 620 reads the N burst or the Q burstbased on the timing T4A3 at which the servo mark is detected.

FIG. 4B is a diagram illustrating an example of a process ofdemodulating a particular short servo SSV of a particular track TRn. Theshort servo SSV illustrated in FIG. 4B corresponds to the servo regionSV located immediately after the normal servo NSV illustrated in FIG.4A. FIG. 4B illustrates a servo gate (short SG) that demodulates part ofservo data written in the servo region SV. The short SG rises at thestart timing T4B1 corresponding to the leading end of the N burst andfalls at the timing T4B2 corresponding to the trailing end of theadditional pattern.

The demodulation unit 620 sets the start timing T4B1 based on, forexample, the timing T4A3 of the Servo Mark Found illustrated in FIG. 4A.For example, the demodulation unit 620 sets the timing T4B1, which is aparticular time after the timing T4A3 illustrated in FIG. 4A, as thestart timing for starting the demodulation of the short servo SSV. Thedemodulation unit 620 positions the read head 15R at the particulartrack TRn based on the preamble, the servo mark, the gray code, and thelike of the normal servo NSV of the track TRn read immediately before,starts the reading of the short servo SSV at the start timing T4B1,positions the read head 15R at the target servo demodulation position ofthe particular track TRn, reads and demodulates the N burst, the Qburst, and the additional patterns in this order, and ends the readingof the short servo SSV at the end timing T4B2 at which the additionalpattern is read. For example, the demodulation unit 620 sets the endtiming T4B2 based on the time for reading the N burst, the Q burst, andthe post code of the normal servo NSV illustrated in FIG. 4A, the starttiming T4B1, the length of the post code, the length of the additionalpattern. As one example, the demodulation unit 620 sets the timing T4B2after the time corresponding to a difference between the time forreading the N burst, the Q burst, and the post code of the normal servoNSV illustrated in FIG. 4A and the time corresponding to a differencebetween the length of the post code and the length of the additionalpattern, from the start timing T4B1, as the end timing for ending thereading of the short servo SSV. Note that the demodulation unit 620 maybe capable of selecting whether to demodulate the particular servoregion SV with the normal SG or the short SG. Hereinafter, the processof demodulating the normal servo NSV and the short servo SSV of theservo region SV with the normal SG may be referred to as a normal servomode, and the process of demodulating the normal servo NSV of the servoregion SV with the normal SG and demodulating the short servo SSV of theservo region SV with the short SG may be referred to as a short servomode. For example, in the normal servo mode, the demodulation unit 620demodulates the preamble, the servo mark, the gray code, the PAD, the Nburst, the Q burst, and the post code of the normal servo NSV, anddemodulates the preamble, the servo mark, the gray code, the PAD, the Nburst, the Q burst, and the additional pattern of the short servo SSV.For example, in the short servo mode, the demodulation unit 620demodulates the preamble, the servo mark, the gray code, the PAD, the Nburst, the Q burst, and the post code of the normal servo NSV, anddemodulates the N burst, the Q burst, and the additional pattern of theshort servo SSV.

For example, the demodulation unit 620 may perform demodulation by usingthe normal servo mode during the seek operation, and may performdemodulation after switching to the short servo mode before performingthe data write/read operation after the seek.

FIG. 5 is a diagram illustrating an example of a process of demodulatingthe N burst, the Q burst, and the additional pattern of the short servoSSV in the short servo mode. The short servo SSV illustrated in FIG. 5corresponds to the short servo SSV illustrated in FIG. 4B. FIG. 5illustrates a short SG, a gate (burst gate: BG) for demodulating burstdata (N burst and Q burst), and a gate for demodulating an additionalpattern (additional pattern read gate: additional pattern RG). The BGrises at a start timing T511 corresponding to the leading end of the Nburst, falls at an end timing T512 corresponding to the trailing end ofthe N burst, rises at a start timing T513 corresponding to the leadingend of the Q burst, and falls at an end timing T514 corresponding to thetrailing end of the Q burst. In addition, the additional pattern RGrises at a start timing T521 corresponding to the leading end of theadditional pattern, and falls at an end timing T522 corresponding to thetrailing end of the additional pattern.

The demodulation unit 620 positions the read head 15R at the particulartrack TRn based on the preamble, the servo mark, the gray code, and thelike of the normal servo NSV of the track TRn read immediately before,starts the reading of the N burst at the start timing T511 set based onthe gray code and the like of the normal servo NSV read immediatelybefore, reads the N burst, performs demodulation by, for example,discrete Fourier transform, to calculate the phase and amplitude of theN burst, ends the reading of the N burst at the end timing T512, startsthe reading of the Q burst at the start timing T513, reads the Q burst,performs demodulation by, for example, discrete Fourier transform, tocalculate the phase and amplitude of the Q burst, and ends the readingof the Q burst at the end timing T514. Note that, instead of the phaseand the amplitude, a sin component and a cos component of the N burst orthe Q burst may be calculated by discrete Fourier transform or the like.The demodulation unit 620 starts the reading of the additional patternat the start timing T521, reads the additional pattern, performsdemodulation by, for example, discrete Fourier transform or the like tocalculate the phase of the additional pattern, and ends the reading ofthe additional pattern at the end timing T522. Note that, in addition toreading the additional pattern and calculating the phase of theadditional pattern by discrete Fourier transform or the like, theamplitude may be simultaneously calculated. In addition, instead of thephase and the amplitude, a sin component and a cos component may becalculated.

When the short servo SSV is demodulated in the short servo mode, thedemodulation unit 620 does not read the preamble, the servo mark, thegray code, and the like of the short servo SSV. Therefore, there is apossibility that the servo demodulation position (servo circumferentialposition) may deviate in the circumferential direction with respect tothe target servo demodulation position, for example, the target servocircumferential position because the start timing T4B1 or the end timingT4B2 of the short servo SSV deviate from the target timing. In addition,when the short servo SSV is demodulated in the short servo mode, thedemodulation unit 620 does not read the preamble, the servo mark, thegray code, and the like of the short servo SSV. Therefore, there is apossibility that the servo demodulation position (servo radial position)may deviate in the radial direction with respect to the target servodemodulation position, for example, the target servo radial position.

The correction unit 630 corrects the servo demodulation position. Forexample, the correction unit 630 corrects the servo demodulationposition of the short servo SSV based on the phase of the N burst, thephase of the Q burst, and the phase of the additional patternrespectively acquired by demodulating the N burst, the Q burst, and theadditional pattern of the short servo SSV. When it is determined thatthe phase of the additional pattern deviates by a particular value(threshold value) or more with respect to a reference phase (hereinafterreferred to as a reference additional pattern phase), the correctionunit 630 determines that the servo circumferential position deviateswith respect to the target servo circumferential position because theread timing of the short servo SSV deviates, corrects the servodemodulation position (radial position) of the short servo SSV so as tocorrect the servo demodulation position (radial position) by correctinga wrong servo demodulation position (radial position). In other words,when it is determined that the absolute value of (the phase of theadditional pattern−the phase of the reference additional pattern)≥thethreshold value, the correction unit 630 determines that the read timingof the short servo SSV, for example, the read timings of the N burst,the Q burst, and the additional pattern deviate, and corrects the servodemodulation position (radial position) of the short servo SSV.

FIG. 6 is a diagram illustrating an example of a phase of eachadditional pattern corresponding to each circumferential position of aparticular track demodulated in a normal servo mode and a phase of eachadditional pattern corresponding to each circumferential position of aparticular track demodulated in a short servo mode. In FIG. 6, ahorizontal axis represents a circumferential direction, and a verticalaxis represents a phase [dibit]. FIG. 6 illustrates a phase (hereinafterreferred to as a phase group of additional patterns in the normal servomode) NMG of each additional pattern corresponding to eachcircumferential position of a particular track demodulated in the normalservo mode, a phase (hereinafter referred to as a phase group ofadditional patterns in the short servo mode or simply as a phase groupof additional patterns) SMG of each additional pattern corresponding toeach circumferential position of a particular track demodulated in theshort servo mode, and a reference additional pattern phase Ref. Thereference additional pattern phase Ref is set based on a phasecalculated by demodulating a plurality of additional patternsrespectively written to a plurality of servo regions of a plurality oftracks. For example, the reference additional pattern phase Ref is setbased on the average value of the phase group NMG of the additionalpatterns in the normal servo mode. A part of the phase group SMG of theadditional patterns deviates by 1 dibit, for example, 180°, with respectto the reference additional pattern phase Ref (the phase group NMG ofthe additional patterns in the normal servo mode).

When it is determined that the phase SMP of the additional pattern inthe phase group SMG of the additional patterns deviates by the thresholdvalue or more with respect to the reference additional pattern phaseRef, the correction unit 630 corrects the servo demodulation position(radial position) of the short servo SSV. When it is determined that thephase SMP of the additional pattern in the phase group SMG of theadditional patterns deviates by the threshold value, for example, 0.5 ormore with respect to the reference additional pattern phase Ref, thecorrection unit 630 corrects the servo demodulation position of theshort servo SSV by one servo track. The servo demodulation position(Demodpos) that can be calculated from the N burst and the Q burst iscalculated in the range of ±one servo track. In other words, the servodemodulation position calculated from the N burst and the Q burst is inthe range of ±one servo track with respect to the servo cylinder. Basedon the sign of the servo demodulation position calculated from the Nburst and the Q burst, one servo track subtraction correction isperformed if the sign is positive and one servo track additioncorrection is performed if the sign is negative.

If Demodpos≥0, after Demodpos correction=before Demodpos correction−1

If Demodpos<0, after Demodpos correction=between Demodpos corrections+1

Note that, when it is determined that the phase SMP of the additionalpattern in the phase group SMG of the additional patterns does notdeviate by 0.5 or more with respect to the reference additional patternphase Ref, the correction unit 630 does not correct the servodemodulation position (radial position) of the short servo SSV. Notethat the phase acquired from the additional pattern may be used forinitial phase correction of the N burst and the Q burst or disksynchronous write (DSW) correction. In addition, when the amplitude isacquired from the additional pattern, it may be used for head flyingheight correction.

In addition, when it is determined that the phase of the N burst and thephase of the Q burst of the short servo SSV deviate with respect to areference phase (hereinafter referred to as a reference burst phase) andthe phase of the additional pattern does not deviate with respect to thephase of the reference additional pattern, the correction unit 630determines that the servo radial position of the short servo SSVdeviates with respect to the target servo radial position.

For example, when it is determined that the phase of the N burst and thephase of the Q burst of the short servo SSV are inverted by 180° withrespect to the reference burst phase and the servo radial position issmaller than the target servo radial position, the correction unit 630determines that the servo demodulation position deviates by one trackoutward in the radial direction and reflects the same to the positioncontrol of the next servo sector. Here, the target servo radial positioncorresponds to, for example, the track center of the particular track.For example, the target servo radial position may be 0. For example,when it is determined that the phase of the N burst and the phase of theQ burst of the short servo SSV are inverted by 180° with respect to thereference burst phase and the servo radial position is equal to orgreater than the target servo radial position, the correction unit 630determines that the servo demodulation position deviates by one trackinward in the radial direction and reflects the same to the positioncontrol of the next servo sector. Note that, for example, when it isdetermined that the phase of the N burst and the phase of the Q burst ofthe short servo SSV are inverted by 180° with respect to the referenceburst phase and the servo radial position is smaller than the targetservo radial position, the correction unit 630 may determine that theservo demodulation position deviates by one track inward in the radialdirection and reflect the same to the position control of the next servosector. For example, when it is determined that the phase of the N burstand the phase of the Q burst of the short servo SSV are inverted by 180°with respect to the reference burst phase and the servo radial positionis equal to or greater than the target servo radial position, thecorrection unit 630 may determine that the servo demodulation positiondeviates by one track outward in the radial direction and reflect thesame to the position control of the next servo sector.

FIG. 7 is a diagram illustrating an example of a path of a servodemodulation position in which a servo demodulation position iscorrected in a short servo mode when a head 15 deviates in a radialdirection at a particular circumferential position. In FIG. 7, ahorizontal axis represents a circumferential direction, and a verticalaxis represents an error with respect to a target servo radial position.FIG. 7 illustrates a change (hereinafter referred to as a path) TGR indeviation with respect to the target servo radial position at eachcircumferential position of the servo demodulation position predicted byanalysis or the like when the head 15 deviates in the radial directionat the particular circumferential position, a path NSR of the servodemodulation position acquired by demodulating each short servo SSVspaced apart in the circumferential direction in the normal servo modewhen the head 15 deviates in the radial direction at the particularcircumferential position, and a path SSR of the servo demodulationposition (hereinafter referred to as the corrected servo demodulationposition) acquired when the correction unit 630 corrects the servodemodulation position acquired by demodulating each short servo SSVspaced apart in the circumferential direction in the short servo mode,as described above, when the head 15 deviates in the radial direction atthe particular circumferential position.

As illustrated in FIG. 7, the correction servo demodulation position SSRmatches the path NSR of the servo demodulation position. That is, whenthe correction unit 630 performs correction as described above, it ispossible to demodulate the servo demodulation position (radial position)calculated by demodulating each servo region SV of the particular trackwith the short SG without mistake, and it is possible to match the servodemodulation position calculated by demodulating each servo region SV ofthe particular track with the normal SG.

The pattern writing unit 640 writes the servo region SV to the disk 10in the manufacturing process. For example, the pattern writing unit 640discretely writes the plurality of servo regions SV in the radialdirection of the disk 10 at particular intervals in the circumferentialdirection in the manufacturing process. For example, the pattern writingunit 640 writes the preamble, the servo mark, the gray code, the PAD,the N burst, the Q burst, and the additional pattern in this order toeach servo region SV of the particular track in the circumferentialdirection, and sets each servo region SV of the particular track as theshort servo SSV. The pattern writing unit 640 writes the preamble, theservo mark, the gray code, and the PAD in the radial direction. Thepattern writing unit 640 writes the N burst in the radial directionafter the PAD such that the phase of the N burst is inverted by 180° inone servo track cycle in the radial direction. The pattern writing unit640 writes the Q burst in the radial direction after the N burst suchthat the phase of the Q burst is inverted by 180° in one servo trackcycle in the radial direction. The pattern writing unit 640 writes the Qburst in the radial direction after the N burst such that the phase ofthe N burst and the phase of the Q burst deviate by 90° in the radialdirection. The pattern writing unit 640 writes the additional pattern inthe radial direction after the Q burst such that the phase of theadditional pattern become equal in one servo track cycle in the radialdirection. At the particular track, the pattern writing unit 640overwrites the post code on the additional patterns of the short servosSSV in odd multiples among a plurality of servo region SV, for example,all servo regions SV (short servos SSV) sequentially numbered from 1 inthe circumferential direction, and sets the short servos SSV in oddmultiples among all the short servos SSV sequentially numbered from 1 inthe circumferential direction as the normal servos NSV. Note that, atthe particular track, the pattern writing unit 640 may overwrite thepost codes on the additional patterns of the short servos SSV in evenmultiples among all the short servos SSV sequentially numbered from 1 inthe circumferential direction.

In addition, for example, the pattern writing unit 640 writes thepreamble, the servo mark, the gray code, the PAD, the N burst, and the Qburst in this order in the circumferential direction in each servoregion SV of the particular track. The pattern writing unit 640 mayalternately write the post code and the additional pattern after the Qburst of the entire servo region SV of the particular track. Forexample, the pattern writing unit 640 writes the post codes after the Qbursts of the servo regions SV in odd multiples among all servo regionsSV sequentially numbered from 1 in the circumferential direction, setsthe servo regions SV in odd multiples among all servo regions SVsequentially numbered from 1 in the circumferential direction as thenormal servo NSV, writes the additional patterns after the Q bursts ofthe servo regions SV in even multiples among all the servo regions SVsequentially numbered from 1 in the circumferential direction, and setsthe servo regions SV in even multiples among all the servo regions SVsequentially numbered from 1 in the circumferential direction as theshort servo SSV. Note that the pattern writing unit 640 may write theadditional pattern only after the Q burst of the servo region SV to bedemodulated in the short servo mode.

FIG. 8 is a flowchart illustrating an example of a method of correctinga servo demodulation position according to the present embodiment.

The MPU 60 demodulates the additional pattern to acquire the phase ofthe additional pattern (B801). The MPU 60 calculates the absolute valueof the difference value between the phase of the additional pattern andthe phase of the reference additional pattern (the phase of theadditional pattern−the phase of the reference additional pattern)(B802). The MPU 60 determines whether the difference value between thephase of the additional pattern and the phase of the referenceadditional pattern is equal to or larger than the threshold value orsmaller than the difference value (B803). When it is determined that thedifference value between the phase of the additional pattern and thephase of the reference additional pattern is equal to or larger than thethreshold value (Yes in B803), the MPU 60 corrects the servodemodulation position (B804). For example, when it is determined thatthe difference value between the phase of the additional pattern and thephase of the reference additional pattern is equal to or larger than thethreshold value, the MPU 60 corrects the incorrect servo demodulationposition to the correct servo demodulation position by correcting theservo demodulation position of the short servo SSV, and proceeds to theprocess of B805. By using the servo demodulation position corrected inB804 or the servo demodulation position when it is determined that thedifference value is smaller than the threshold value (No in B803), theMPU 60 combines the servo demodulation position and the radial position(servo cylinder address) of the servo track to calculate the radialposition of the head 15 on the disk 10 (B805), and ends the process.

FIG. 9 is a flowchart illustrating an example of a process of writing anadditional pattern according to the present embodiment.

The MPU 60 writes the preamble, the servo mark, the gray code, the Nburst, and the Q burst in this order in each servo region SV of theparticular track (B901). The MPU 60 writes the additional pattern afterthe Q burst of each servo region SV (B902), and ends the process. Forexample, the MPU 60 writes the additional pattern in the radialdirection after the Q burst such that the phase of the additionalpattern become equal in one servo track cycle in the radial direction.In addition, after writing the additional pattern after the Q burst ofeach servo region SV, the MPU 60 overwrites the post codes on theadditional patterns of the servo regions SV in even multiples among allthe servo regions SV sequentially numbered from 1 in the circumferentialdirection.

FIG. 10 is a flowchart illustrating an example of a process of writingan additional pattern according to the present embodiment.

The MPU 60 writes the preamble, the servo mark, the gray code, the Nburst, and the Q burst in this order in each servo region SV of theparticular track (B901), and determines whether to write the additionalpattern after the Q burst (B1001). When it is determined that theadditional pattern is not to be written (NO in B1001), the MPU 60 endsthe process. When it is determined that the additional pattern is to bewritten (YES in B1001), the MPU 60 writes the additional pattern afterthe Q burst (B1002), and ends the process. For example, the MPU 60writes the additional pattern in the radial direction after the Q burstsuch that the phase of the additional pattern become equal in one servotrack cycle in the radial direction. In addition, the MPU 60 writes theadditional patterns of the servo regions SV in even multiples among allthe servo regions SV sequentially numbered from 1 in the circumferentialdirection.

FIG. 11 is a flowchart illustrating an example of a process of writingan additional pattern according to the present embodiment.

The MPU 60 writes the preamble, the servo mark, the gray code, the Nburst, and the Q burst in this order in each servo region SV of theparticular track (B901), and determines whether to write the additionalpattern after the Q burst (B1001). When it is determined that theadditional pattern is not to be written (NO in B1001), the MPU 60 writesthe post code after the Q burst (B1101), and ends the process. Forexample, the MPU 60 writes the post codes of the servo regions SV in oddmultiples among all the servo regions SV sequentially numbered from 1 inthe circumferential direction, and ends the process. When it isdetermined that the additional pattern is to be written (Yes in B1001),the MPU 60 writes the additional pattern after the Q burst (B1002), andends the process. For example, the MPU 60 writes the additional patternin the radial direction after the Q burst such that the phase of theadditional pattern become equal in one servo track cycle in the radialdirection. In addition, the MPU 60 writes the additional patterns of theservo regions SV in even multiples among all the servo regions SVsequentially numbered from 1 in the circumferential direction, and endsthe process. Note that, after B901 is performed in the servo writingprocess, it may be determined whether to write the additional pattern ofB1001 after proceeding to the next test process.

According to the present embodiment, the magnetic disk device 1 has atleast one short servo SSV between two normal servos NSV continuous inthe circumferential direction in the particular track. The normal servoNSV includes a preamble, a servo mark, a gray code, a PAD, an N burst, aQ burst, and a post code. The frequency of the post code is equal to thefrequency of the preamble. The short servo SSV includes a preamble, aservo mark, a gray code, a PAD, an N burst, a Q burst, and an additionalpattern. The frequency of the additional pattern is different from thefrequency of the preamble and the frequency of the post code. Theadditional pattern has the same phase in one servo track cycle in theradial direction of the disk 10. In other words, the phase of theparticular additional pattern is equal to the phase of the adjacentadditional pattern. The length APL of the additional pattern is shorterthan the length PCL of the post code. The magnetic disk device 1demodulates the normal servo NSV with the normal SG and demodulates theshort servo SSV with the short SG. When the short servo is demodulatedwith the short SG, the magnetic disk device 1 demodulates the additionalpattern to acquire the phase of the additional pattern. The magneticdisk device 1 calculates the absolute value of the difference valuebetween the phase of the additional pattern and the phase of thereference additional pattern. When it is determined that the differencevalue between the phase of the additional pattern and the phase of thereference additional pattern phase is equal to or larger than thethreshold value, the magnetic disk device 1 corrects the servodemodulation position of the short servo SSV. Therefore, the magneticdisk device can acquire the correct servo demodulation position (radialposition) without using the wrong servo demodulation position (radialposition), thereby improving the accuracy of the servo demodulationposition. In addition, since the short servo SSV is demodulated with theshort SG, data can be written up to immediately before the preamble ofthe short servo SSV, and the length APL of the additional pattern isshorter than the length PCL of the post code. Therefore, the magneticdisk device 1 can increase the recording area to which user data can bewritten. Therefore, the magnetic disk device 1 can improve the servoformat efficiency.

Next, a magnetic disk device according to a modification will bedescribed. In the modification, the same reference numerals are assignedto the same parts as those of the above-described embodiment, and thedetailed description thereof will be omitted.

Modification 1

The magnetic disk device 1 of Modification 1 differs from the embodimentdescribed above in the configuration of the short servo SSV.

FIG. 12 is a schematic diagram illustrating an example of theconfiguration of the short servo SSV according to Modification 1.

When user data is written to two servo regions SV continuous in thecircumferential direction, for example, a recording area between thenormal servo NSV and the short servo SSV located after the normal servoNSV, the MPU 60 overwrites the user data on part of servo data not readby the short SG among servo data of the short servo SSV. For example,when the user data is written in the recording area between the normalservo NSV and the short servo SSV located after the normal servo NSV,the MPU 60 overwrites the user data on part of the preamble of the shortservo SSV. Note that, when the user data is written in the recordingarea between the normal servo NSV and the short servo SSV located afterthe normal servo NSV, the MPU 60 may overwrite the user data on thepreamble, the servo mark, the gray code, and the PAD of the short servoSSV.

According to Modification 1, when user data is written to a recordingarea between the normal servo NSV and the short servo SSV located afterthe normal servo NSV, the magnetic disk device 1 overwrites the userdata on part of servo data not read by the short SG among servo data ofthe short servo SSV. Therefore, the magnetic disk device 1 can improvethe servo format efficiency.

While certain embodiments have been described, these embodiments havebeen presented by way of example only, and are not intended to limit thescope of the inventions. Indeed, the novel embodiments described hereinmay be embodied in a variety of other forms; furthermore, variousomissions, substitutions and changes in the form of the embodimentsdescribed herein may be made without departing from the spirit of theinventions. The accompanying claims and their equivalents are intendedto cover such forms or modifications as would fall within the scope andspirit of the inventions.

What is claimed is:
 1. A magnetic disk device comprising: a diskcomprising two first servo sectors arranged side by side in acircumferential direction and at least a second servo sector locatedbetween the two first servo sectors; a head that writes data to the diskand reads data from the disk; and a controller that demodulates all dataof the first servo sector and demodulates data of part of the secondservo sector, wherein the first servo sector comprises burst data and afirst data pattern written before the circumferential direction of theburst data, the second servo sector comprises the burst data, the firstdata pattern written before the circumferential direction of the burstdata, and a second data pattern written after the circumferentialdirection of the burst data, a first frequency of the first data patternis different from a second frequency of the second data pattern, a firstlength in the circumferential direction of the first data pattern isdifferent from a second length in the circumferential direction of thesecond data pattern, and the first data pattern comprises a preamble,and the first frequency is equal to a third frequency of the preamble.2. The magnetic disk device according to claim 1, wherein the secondfrequency corresponds to ½ of the third frequency and is equal to afourth frequency of the burst data.
 3. The magnetic disk deviceaccording to claim 1, wherein the second length is equal to or largerthan the sum of the reciprocal of the second frequency and twice thereciprocal of the third frequency.
 4. The magnetic disk device accordingto claim 1, wherein the first data pattern comprises a servo markwritten after the preamble and a gray code written after the servo mark.5. The magnetic disk device according to claim 1, wherein the controllerdemodulates the first data pattern and the burst data in this order inthe first servo sector, and demodulates the burst data and the seconddata pattern in this order in the second servo sector.
 6. The magneticdisk device according to claim 1, wherein the controller overwrites userdata on the first data pattern of the second servo sector.
 7. A magneticdisk device comprising: a disk comprising two first servo sectorsarranged side by side in a circumferential direction and at least asecond servo sector located between the two first servo sectors; a headthat writes data to the disk and reads data from the disk; and acontroller that demodulates all data of the first servo sector anddemodulates data of part of the second servo sector, wherein the firstservo sector comprises burst data and a first data pattern writtenbefore the circumferential direction of the burst data, the second servosector comprises the burst data, the first data pattern written beforethe circumferential direction of the burst data, and a second datapattern written after the circumferential direction of the burst data, afirst frequency of the first data pattern is different from a secondfrequency of the second data pattern, a first length in thecircumferential direction of the first data pattern is different from asecond length in the circumferential direction of the second datapattern, and the first servo sector comprises a third data patternwritten after the circumferential direction of the burst data, a fifthfrequency of the third data pattern is different from the secondfrequency of the second data pattern, and the third length in thecircumferential direction of the third data pattern is different fromthe second length in the circumferential direction of the second datapattern.
 8. The magnetic disk device according to claim 7, wherein thesecond length is shorter than the third length.